How Does Your Gas Gauge Really Work?

Why That First Half-Tank Seems To Last Forever

Today's cars are packed with high-technology features throughout. From the latest overhead cam, turbocharged, direct-injected engines, to slick-shifting, computer-controlled twin-clutch transmissions to voice-controlled infotainment systems, there is virtually no area of a modern car that has not benefited in some way from these advances. Except for maybe the gas gauge, that is.

With a plethora of integrated, microprocessor-controlled systems throughout the car, delivering countless calculations per second regarding speed, throttle position, temperature, fuel mixture, the car's attitude to the road – even navigation systems that can pinpoint your location within 100 feet no matter where you are on the planet – the ordinary gas gauge continues to give you, at best, a vague estimate of how much fuel you have in the tank.

Giving The Customer What They Want
Have you ever noticed that your gas gauge stays on full for quite a while before the needle even moves and then it moves faster and faster as it approaches empty? And then when it gets to "E" it sort of stays there for a while until the low warning light comes on, which might even be accompanied by a friendly chime, just in case you didn't get the visual memo that it's time to fill up?

It turns out it's partially your fault that gas gauges work that way. The engineers calibrate them to do that. Why? Because you, the customer, have told them that's the way you like it. We spoke with Phil Pierron, an engineer at Ford (his title is actually "Technical Expert for Systems Engineering in Core Fuel Systems), who told us, "Our customers really didn't want to run out of fuel when they hit 'E'. Customers do want some amount of fuel when they get to 'E'."

Apparently, consumer surveys indicate that people don't like seeing the needle depart from 'F' right away either, which it should technically do the moment you leave the gas station. Again, according to Pierron, "[Customers] want it to say on full for an amount of time." This gives them the illusion that they are getting better fuel mileage or at least not immediately burning through that expensive tank of petrol they just bought, even if they quite literally are. And just as you have a 'reserve' that kicks in after you hit 'E', the engineers call this stickiness on the 'F' mark a 'full reserve'.

Whatever Happened To Accuracy?
If fans of accuracy were giving out grades, they just might give that attitude the same 'F'. The very idea of calibrating a gas gauge to assuage our feelings toward our car is akin to setting your clock ahead 10 minutes so that even though you know it's 10 minutes fast, it somehow motivates you to get out of bed earlier, when in reality all it does is encourage you to snooze away that extra ten minutes anyway. The difference with the gas gauge is that those 10 extra minutes are not optional on the driver's part. We are stuck with gas gauges designed to make people feel better, not accurately report the level of fuel in the car.

While customers want there to be a reserve of gasoline available when they reach the empty mark, there isn't technically a reserve tank, there's simply a real empty point that is not marked on the gauge. At the same time customers don't want too much of a reserve. Otherwise, they will complain that their 20-gallon tank only takes 15 gallons when filling up from empty. Apparently, there is a sweet spot where customers are happy to be fooled by their gas gauges, but not too much. We customers sure are a fickle bunch.

The engineer's job should be to make things more accurate and efficient, but in this case he has to play psychologist to keep customers happy. In defense of the engineers, gas gauges do need a bit more calibrating these days since the tanks, particularly on passenger cars, are often oddly shaped to fit around the various parts and fill spaces in their allotted areas underneath the chassis or within the unibody. They also have to meet crash, temperature and emissions requirements.

The Un-measurable Top-Off
Santo Spadaro, an experienced mechanic who grew up in the family business (Dominick's European Car of White Plains, New York), doesn't necessarily see the gauge as inaccurate, but with an impossible task in determining what's full. Spadaro points out that "beyond the gas tank, if you top it off you end up with three-quarters of a gallon, maybe a full gallon, of extra fuel [in the filler neck]. So, it seems that first quarter of a tank you are getting sensational fuel economy." He also points out that he usually tops off his tank with a few extra pulls of the gas pump even after it reaches the automatic stopping point because he likes to keep those trips to the gas station to a minimum.

Ford engineer Pierron also has a difficult task on his hands when calibrating for that top-off routine, as it can be difficult to accurately measure. All gas pumps are not created equal, so one pump might give you a few more tenths before shutting off than the next pump. And some customers may pull the nozzle when it stops the first time, while others may keep going until the gas is splashing out onto the ground.

Calibration also helps the gauge stay steady when you are on a steep incline, on a twisty road or when you park the car in an unusual position. Even with a half tank of gas, you might see full or empty the next morning when stepping into the car if the gauge is not calibrated to factor the car's attitude into the equation.

Old School Technology
Curiously, while the technology clearly exists today to give every driver a very accurate reading of the fuel in the tank, the actual mechanism that measures the fuel in the tank has changed very little in the past few decades. Spadaro, at whose shop you are just as likely to see a classic Alfa Romeo or rare Lancia from the 1960s as you are a modern BMW or Jaguar, has seen them all and agrees that the sending unit really hasn't changed much. "Generally, they're all created equal and they're all variations. It's a lighter-than-gas float and a rheostat," says Spadaro.

In the old days of simple electro-mechanical gauges, the accuracy of the gauge was usually thrown off when the tank was near full and the float might be submerged for some amount of time as the fuel gets used or when the tank was near empty as the float might reach its lowest point even as various nooks and crannies in the tank might still have gas in them. Today's microprocessor-controlled gas gauges could easily clear up those discrepancies, given the engineer's ability to program them out when calibrating the gauge.

Here's an idea for the next-generation of fuel gauges, particularly now that instrument clusters are largely being replaced by single-panel LCD units: Give the driver the choice. How about a button to switch between an accurate readout and the "feel good" calibration? And then let it sync the dash clock to ten minutes faster at the same time.